There is no set time. Too many variable's. Good insurance. Guys that fish a lot= every year, others every other year. Watch your water pressure.
10-4 , blown them in one month( alligator weed clearing)
have had one go seven years (using boat about every day )
water pressure + temp gauge works good. (merc 2.0 and 2.5)
lots of folks don't believe in temp gauge , however you can keep a eye on your thermo's.
water pressure tells you about the pop-off valve ..(If you understand the mechanics of it )